The present invention relates generally to catalytic bodies and more specifically to catalytic bodies for use as anodes in electrolytic cells. The anodes provide low overvoltage, fast kinetics, chemical stability, good electrical conductivity, low heat of oxygen adsorption and good mechanical strength.
The electrolytic decomposition of water in an alkaline electrolyte has long been practiced for the production of hydrogen gas. The major components of the cell in which such electrolysis takes place usually includes an anode and a cathode which are in contact with an electrolytic solution, and a diaphragm or membrane separator in the cell to separate the anode and cathode and their reaction products. In operation, the selected electrolyte, such as NaOH, KOH or H.sub.2 SO.sub.4 for example, is continually fed into the cell and a voltage is applied across the anode and cathode. This produces electrochemical reactions which take place at the anode and cathode to form oxygen and hydrogen gas, respectively. These reactions and the overall reaction is represented as follows:
______________________________________ Cathode: 2H.sub.2 O + 2e.sup.- .fwdarw. H.sub.2 + 2OH.sup.- Anode: 2OH.sup.- .fwdarw. 1/2 O.sub.2 + 2e.sup.- + H.sub.2 O Total: H.sub.2 O .fwdarw. H.sub.2 + 1/2 O.sub.2 ______________________________________
The particular materials utilized for the anode and cathode are important since they respectively provide the necessary catalysts for the reactions taking place at the anode and cathode. For example, the role which the anode catalyst M is believed to play in evolving oxygen in an electrolytic cell is as follows: EQU M+OH.sup.- .fwdarw.MOH+e.sup.- EQU MOH+OH.sup.- .fwdarw.MO+H.sub.2 O+e.sup.- EQU 2MO.fwdarw.MO.sub.2 +M EQU MO.sub.2 .fwdarw.O.sub.2 +M
In addition to allowing the desired reactions to take place, the catalytic efficiency of the catalytic materials is a very important consideration since an efficient catalytic material reduces the operating energy requirements of the cell. The applied voltage necessary to produce the anode and cathode reactions in an electrolytic cell is the sum of the decomposition voltage (thermodynamic potential) of the compounds in the electrolyte being electrolized, the voltage required to overcome the resistance of the electrolyte and the electrical connectors of the cell, and the voltage required to overcome the resistance to the passage of current at the surface of the anode and cathode (charge transfer resistance). The charge transfer resistance is referred to as the overvoltage. The overvoltage represents an undesirable energy loss which adds to the operating costs of the electrolytic cell.
The reduction of the overvoltage at the anode and cathode to lower operating cost of the cell has been the subject of much attention in the prior art. More specifically, as related to this invention, considerable attention has been directed at the reduction of overvoltage caused by the charge transfer resistance at the surface of the anode due to catalytic inefficiencies of the particular anode materials utilized.
The anode overvoltage losses can be quite substantial in electrolytic cells. For example, for nickel anodes or nickel plated steel anodes, the materials most commonly used by the water electrolysis industry, the charge transfer resistance is on the order of 400 mV at one set of typical operating conditions, e.g., a 30% KOH electrolyte at a temperature of 80.degree. C. and current density of 2 KA/m.sup.2. Because such cells are used to annually produce a significantly large amount of hydrogen, the total electrical energy consumed amounts to a very substantial sum in view of the high electrical energy cost. Such a large amount of energy is consumed that even a small savings in the overvoltage such as 30-50 mV would provide a significant reduction in operating costs. Furthermore, due to the trend of rapidly rising costs for electrical energy, the need for reduced overvoltages takes on added importance since the dollar value of the energy to be saved continually is increasing.
One reason nickel and nickel plated steel catalytic materials have been most commonly used for the electrolysis of water is because of their relatively low cost. Another reason is that these materials are resistant to corrosion in hot concentrated caustic solutions and have one of the lowest overvoltages among the non-noble metal materials for the oxygen evolution reaction. Nickel and nickel plated steel, however, as discussed above, are not particularly efficient catalysts and thus operate with considerable overvoltages. Nevertheless, the excessive overvoltages provided by nickel and nickel plated steel anodes have been reluctantly tolerated by the industry since an acceptable alternative anode material has not been available and the cost of electrical power until recently was not a major cost consideration.
A limitation in the efficiency of nickel anodes, as well as many other materials proposed for use as a catalytic material for anodes for an electrolytic cell, is that these materials are single phase or substantially single phase crystalline structures. In a single phase crystalline material the catalytically active sites which provide the catalytic effect of such materials result from accidently occurring, surface irregularities which interrupt the periodicity of the crystalline lattice. A few examples of such surface irregularities are dislocation sites, crystal steps, surface impurities and foreign absorbates.
A major shortcoming with basing the anode materials on a crystalline structure is that irregularities which result in active sites typically only occur in relatively few numbers on the surface of a single phase crystalline material. This results in a density of catalytically active sites which is relatively low. Thus, the catalytic efficiency of the material is substantially less than that which would be possible if a greater number of catalytically active sites were available for the oxygen or other gas evolution reaction at the anode. Such catalytic inefficiencies result in overvoltages which add substantially to the operating costs of the electrolytic cells.
One prior art attempt to increase the catalytic activity of the anode was to increase the surface area of the cathode by the use of a "Raney"-type process. Raney nickel production involves the formation of a multi-component mixture, from melted or interdiffused components such as nickel and aluminum, followed by the selective removal of the aluminum, to increase the actual surface area of the material for a given geometric surface area. The resulting surface area for Raney nickel anodes is on the order of 100-1000 times greater than the geometric area of the material. This is a greater surface area than the nickel and nickel plated steel anodes discussed above.
The Raney nickel anodes are very unstable and lack mechanical stability during gas evolution. The degradation reduces the operating life of Raney nickel anodes and thus they have not been widely accepted for industrial use. Furthermore, the process for producing Raney nickel is relatively costly due to the expense of the various metallurgical processes involved.
Many other anode materials have been prepared and tested at least on an experimental basis. For various reasons, however, these materials have not replaced nickel and nickel plated steel anodes as the most commonly used industrial anode materials. Some of these experimentally prepared anode materials include mixtures of nickel and other metals. The preparations have varied and include plasma spraying a mixture of cobalt and/or nickel along with stainless steel onto a nickel or nickel coated iron substrate; subjecting a nickel molybdate material to a anodic polarization procedure to remove the molybdenum therefrom to form a finely divided nickel oxide; nickel sinters impregnated with precipitated nickel (II) hydroxide; and a spinel NiCo.sub.2 O.sub.4 material prepared as a powder by freeze drying or by co-precipitation from a solution of mixed salts.
Another prior art approach to lower the overvoltage of anode catalysts has been centered around the use of materials which are inherently better catalysts than nickel. Certain compositions including noble metals can provide catalysts which exhibit lower overvoltages during utilization as an anode catalyst, but these materials have other major drawbacks which have prevented a widespread acceptance by industrial users of electrolytic cells. These materials are much too expensive for efficient commercial use, are relatively scarce and are usually obtained from strategically vulnerable areas. Another drawback is that once placed into operation in an electrolytic cell, further degradation problems arise since the noble metal including materials are quite susceptible to "poisoning".
Poisoning occurs when the catalytically active sites of the material become inactivated by poisonous species invariably contained in the electrolytic solution. These poisonous species may, for example, include residual ions contained in untreated water used in the electrolyte such as ions of the normal impurities found in water, Ca, Mg, Fe and Cu. Once inactivated such sites are thus no longer available to act as a catalyst for the desired reaction and catalytic activity is reduced increasing the overvoltage losses.
In summary, various catalytic materials for use as electrolytic cell anodes have been proposed. Nickel and nickel plated steel anodes have been most commonly commercially used. These materials are catalytically inefficient resulting in considerable overvoltages which add significantly to operating costs. Those materials which exhibit lower overvoltages, such as noble metal including catalysts, are expensive and/or subject to poisoning. Other anode materials which exclude noble metals have been proposed, but it appears that such materials do not improve the overall anode performance in terms of overvoltage savings, material costs and operating life since such prior art anodes have not been accepted to any significant degree. Thus, there remains the need for a stable, low overvoltage anode material of low cost to replace the presently used catalytic materials for oxygen evolution in an electrolytic cell.